Epidemiology of sleeping sickness in Boffa (Guinea): where are the trypanosomes?

Abstract

Human African Trypanosomiasis (HAT) in West Africa is a lethal, neglected disease caused by Trypanosoma brucei gambiense transmitted by the tsetse Glossina palpalis gambiensis. Although the littoral part of Guinea with its typical mangrove habitat is the most prevalent area in West Africa, very few data are available on the epidemiology of the disease in such biotopes. As part of a HAT elimination project in Guinea, we carried a cross-sectional study of the distribution and abundance of people, livestock, tsetse and trypanosomes in the focus of Boffa. An exhaustive census of the human population was done, together with spatial mapping of the area. Entomological data were collected, a human medical survey was organized together with a survey in domestic animals. In total, 45 HAT cases were detected out of 14445 people who attended the survey, these latter representing 50.9% of the total population. Potential additional carriers of T. b. gambiense were also identified by the trypanolysis test (14 human subjects and two domestic animals). No trypanosome pathogenic to animals were found, neither in the 874 tsetse dissected nor in the 300 domestic animals sampled. High densities of tsetse were found in places frequented by humans, such as pirogue jetties, narrow mangrove channels and watering points. The prevalence of T. b. gambiense in humans, combined to low attendance of the population at risk to medical surveys, and to an additional proportion of human and animal carriers of T. b. gambiense who are not treated, highlights the limits of strategies targeting HAT patients only. In order to stop T. b. gambiense transmission, vector control should be added to the current strategy of case detection and treatment. Such an integrated strategy will combine medical surveillance to find and treat cases, and vector control activities to protect people from the infective bites of tsetse.

Figure 1. Setting up a floating trap in a mangrove channel.

Figure 2. Human settlement and animal sampling in Boffa.

The human settlement in Boffa is defined by 123 settlements comprising villages, hamlets and encampments harbouring variable population densities. Main places are located on the mainland (Yenguissa, Bamba, Kaonso) but also on some islands (Yangoya, Koteya). In general, islands are less populated than mainland, and the West bank of Rio Pongo (especially the islands part) shows a very low human settlement compare to the East bank. On the islands, there are a lot of small temporary (seasonal) settlements for salt extraction, fishing and rice cultivation. Most of the people on the mainland, who have activities on islands, use pirogue to go to the islands through mangrove channels. The location of the animals sampled in the area is also shown.

Figure 3. Geographic location of bridges, watering points and pirogue jetties in the focus of Boffa.

All the bridges of the area are built on small rivers coming from the mainland and going towards mangrove ecosystem. The mangrove channels are the continuity of these little rivers. The high human mobility between mainland and islands is characterized by the 129 recorded and georeferenced pirogue jetties. The location of the 404 watering points (pumps, holes, springs, river points) are concentrated on the mainland with a very low number on the islands, especially on the West bank of Rio Pongo. All these points represent potential contact between humans and tsetse.

Figure 4. Location of traps and densities of tsetse caught during the entomological survey.

On the mainland, tsetse (Glossina palpalis gambiensis) are found mainly along rivers and water points. In the mangrove, main densities are found on narrow mangrove channels and around frequented places such as pirogue jetties.

Figure 5. Spatial distribution of trypanosomes in humans, animals and tsetse.

T. b. gambiense is found in HAT cases (here represented by the location of their house) and in both the seropositives-TL+ and two of the animals sampled. The trypanosomes observed parasitologically in tsetse could not be identified and are probably not T. b. gambiense (see text for details).